1. Field of the Invention
The present invention relates to digital imaging systems, and more particularly, to a method and apparatus for serving high resolution and high frame rate image data files to a digital display using a standard lower resolution server.
2. Description of Related Art
The concept of “digital cinema” includes the production, delivery and presentation of audio/visual material in auditoriums or theatres using digital technology. A digitally produced or digitally converted movie is typically distributed in a compressed and encrypted form on physical media such as DVD-ROM, tape or computer hard drives and can, in principle, be distributed by electronic transmission using satellite or other broadband communication paths.
Digital cinema systems are often used in commercial movie theaters in order to play back and display feature films at a quality comparable with that of 35 mm film release prints. Such systems typically comprise a digital video display device, such as a digital video projector, for screening of digitized audiovisual content, and a playback server that feeds digital content to the display device. Content is supplied from the server to the display device over one or more video links, such as a dual-link High-Definition Serial Digital Interface (HD-SDI) in order to provide the bandwidth necessary to transfer higher resolution content from the playback server to the display device.
Storage and media block are two components of the theater playback system. Storage is the image server that holds the packaged content for eventual play back. The media block is the hardware device (or devices) that converts the packaged content into the streaming data that ultimately turns into the pictures and sound in the theater. These two components can be physically combined together in an integrated system or the components can be physically separate from each other. The term “server” will be used to indicate both storage and media block contained in a single device.
Current digital cinema systems support 2K (2048×1080) resolution images. There is currently a push to move to higher resolutions such as 4K (4096×2160) in order to provide a final product that is true to the original material. However, there are technical challenges to implement such systems as will be described in more detail.
FIG. 1 illustrates the dimensions of a 2K resolution image 10, while FIG. 2 illustrates the dimensions of a 4K resolution image 12. A 2K image 10 is comprised of an array of pixels, 2048 pixels wide by 1080 pixels high. A 4K image 12 is comprised of an array of pixels, 4096 pixels wide by 2160 pixels high. The dimensions of a 4K resolution image 12 correspond to four times the dimensions of a 2K resolution image 10. Therefore, a 4K resolution image 12 can be cut into four 2K resolution images: one top left 2K image 20, one top right 2K image 22, one bottom left 2K image 24, and one bottom right 2K image 26, as illustrated in FIG. 3. The source image is partitioned into rectangular non-overlapping blocks in a process called tiling. Each tile is 2048 pixels wide by 1080 pixels high. Each tile is then compressed independently as though the tile was an entirely independent image.
Digitized movies are typically distributed from production companies as a compressed 4K resolution image according to the specifications of Digital Cinema Initiatives (DCI). DCI is a joint venture of Disney, Fox, MGM, Paramount, Sony Pictures Entertainment, Universal and Warner Bros. Studios, which documents voluntary specifications for an open architecture for digital imaging. The compressed image is then typically stored in the digital imaging system for later viewing.
DCI specifies that 4K resolution images are to be delivered compressed and the compression standard used should be JPEG 2000 (Joint Photographic Experts Group). The JPEG 2000 algorithm compresses the image by generating a 2K resolution uncompressed image along with compressed higher resolution enhancement components (HR components). FIG. 4 illustrates that a 4K resolution image 12 can be compressed to create an equivalent 2K resolution image 10 along with HR components 28. The 4K resolution image 12 can be reconstituted directly from the 2K resolution image 10 with HR components 28. Once compressed, the 4K higher resolution image 12 can later be reconstructed by combining the 2K resolution image 10 and the compressed HR components 28 together.
Two methods are currently used to display the stored 4K resolution images in the digital cinema system. In a first method, illustrated in FIG. 5, each compressed 4K resolution image that is received is reconstituted into an uncompressed 4K resolution image and is subsequently cut into four tiles: the top left tile 30, the top right tile 32, the bottom left tile 34, and the bottom right tile 36. Each tile is then stored on one of four 2K servers 38a-38d. The result is audiovisual content made of four images. Each image is transferred from a separate 2K image server 38a-38d connected to a digital 4K projector 40 via individual 2K links. As separate servers are used, the images must be synchronized with one another using a time code source signal so that the projector can synchronize the images together to prevent misalignment of frames.
The first method is not optimal as the four image servers 38a-38d have to be synchronized with each other to ensure the tiles play simultaneously. The first method is also costly as the method requires four 2K servers 38a-38d in addition to a special process used to generate the four tiles. As a result, this method is generally not deployed in theatres due to its high complexity and cost.
In a second method, shown in FIG. 6, the digitized audiovisual content is stored on a 4K server 42 as a single package that contains the 2K resolution image and the 4K compressed high resolution component. The 4K server 42 reconstructs the 4K resolution image from the compressed components and then internally separates the image into the four tiles 30, 32, 34, 36 to be transmitted to the projector 40. This second method offers the advantage of storing the audiovisual content in a simple package; however, its disadvantage is in the cost of the server, which is significantly higher than the cost of the more standard 2K servers available on the market. 4K servers are a much newer technology with fewer models commercially available than the standard 2K servers and requiring higher storage capacities, which drives up the cost of the server. In addition, the connection still requires four physical 2K links to transport the 4K resolution image to the projector 40.
As digital imaging systems move towards providing 4K content from the server to the display device, alternate imaging display processes, such as “Digital 3-D” are making the same move to provide higher image quality. “Digital 3-D” is a term used in digital imaging to describe stereoscopic images, where alternating left and right images are displayed on a 2-D screen. A method for viewing these images is employed such that each eye only sees the intended images. The result is that the left eye sees a slightly different image from the right eye, providing the stereoscopic effect.
Stereoscopic content is currently supplied over a dual-link HD-SDI connection as a 2K resolution 4:2:2 10-bit quality signal. Chroma subsampling in a video system is usually expressed as a three part ratio (I.e. 4:2:2). The three terms of the ratio are: the number of brightness samples, followed by the number of samples of the two color components for each complete sample area. For 4:2:2 subsampling, each of the two color-difference channels has half the sample rate of the brightness channel, so horizontal color resolution is only half that of 4:4:4 subsampling. To view stereoscopic content today, a stereoscopic display device is typically used in conjunction with a server similar to that of current 2K systems. The stereoscopic image is sent at a higher frame rate (e.g., 48 frames per second) than typically used for standard content (e.g., 24 frames per-second) and is separated into left eye and right eye components and then interleaved into one data stream. The data stream is then transmitted to the display device that actually displays two images. Special glasses, such as polarized or liquid crystal display (LCD) glasses, cause one of the images to enter one eye and the other to enter the other eye.
Similarly, high frame rate non-stereoscopic images, such as 48 frame-per-second (fps) content, have the same limitations as stereoscopic content and can only be supplied currently as a 2K resolution 4:2:2 10-bit quality signal over a dual-link HD-SDI connection. In order to provide better image quality for non-stereoscopic high frame rate content and stereoscopic content, it would be advantageous to provide a higher quality, high frame rate system that allows for a 2K resolution 4:4:4 12-bit quality signal.
There is a need for a way to leverage existing 2K server technology in order to reduce costs and allow a single server to support both 2K and 4K display. Likewise, there is a need to provide higher frame rate images while utilizing the same 2K server technology.